This invention relates to ball valves and, more particularly, to trunnion type ball valves.
Trunnion type ball valves are well known and generally comprise a valve body having inlet and outlet flow passages and a central cavity for receiving a ball valve member. The ball valve member is formed with a bore and a pair of trunnions extending from opposite sides of the ball along a line generally transverse to the bore. By use of the trunnions, the ball valve is rotatably mounted in the central cavity so that it rotates between an open position where the bore is aligned with the passages and a closed position where the ball valve surface extends across the flow passages. Between the ball valve and the portions of the body surrounding the passages, there is located a valve seat member that cooperates with the ball valve surface to prevent flow of fluid through the valve.
Most ball valves of the trunnion type generally include some biasing arrangement for compressing the valve seats against the ball valve member to provide a fluid tight seal between the seats and the ball valve surface when the valve is in its closed position. These biasing arrangements commonly provide for the compressive force even in the open position of the ball valve. This has resulted in extensive wear on the valve seat or on the ball valve surface during the opening and closing movement of the ball valve because of the compressive force. After a while this wear causes the valves to leak and require frequent replacement of the ball valve and/or the valve seats. In addition, use of the biasing mechanism in the first instance adds to the expense of the ball valve and these mechanisms are another source of failure that necessitates replacement. This, of course, adds to the manufacturing and maintenance costs of the valve.
One attempt to avoid the use of the biasing mechanism is disclosed in U.S. Pat. No. 3,410,523 issued to W. Kelly et al on Nov. 12, 1968. In the Kelly patent, there is disclosed a trunnion type ball valve wherein the valve seat faces are not coaxial with the axis of the inlet and outlet flow passages and wherein the ball is not a true spherical surface. The valve seat members are, in fact, offset to either side of the axis of the flow passages to cooperate with opposed eccentric curved portions of the eccentrically shaped ball valve member. With the ball valve disclosed in the Kelly patent, there is no loading or compression of the valve seat in the open position of the ball valve and only a light loading in the closed position. With this type of arrangement, the light loading in the closed position may be insufficient to establish a satisfactory seal and may not satisfactorily eliminate leakage. In addition, the cost of machining the eccentric shape of the ball valve member adds to the manufacturing cost.
Another problem associated with the ball valve disclosed in the Kelly patent results from the self-compensating feature wherein wear of the valve seats is accommodated by merely turning the ball valve member an additional one degree (1.degree.) to three degrees (3.degree.) to obtain the desired seating. This means that the valve cannot be used with conventional valve actuators which operate to rotate the valve through only ninety degrees (90.degree.) from the closed to the open position. Conventional valve actuators are not torque dependent and would not permit the self-compensation feature to be effectuated.